How would I answer this?
Rather than give you the answer, I’ll try to help you through the steps. You might also find it useful to play with this Phet simulation to help you understand how charges interact.
First, draw a diagram of the oil drop. How many forces are acting on it? What names would you give to these forces?
What size must the force vectors (arrows) be in order for the oil drop to be stationary?
The positively charged plate is above the drop, the negatively charged plate is below. What must the charge on the drop be to remain stationary?
Give that a bit of thought and let me know your answers. I’ll help further if you need it.
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weight force due to gravity would be acting downwards, and the electric force is also acting downwards, so the oil drop would have to be negatively charged?
You’re correct about weight acting downwards. In order for the drop to be stationary (a constant velocity of zero), what do you know about the forces acting on it? (Newton’s 1st Law)
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there must be an equal and opposite force, and there would be no net force because an object at rest stays at rest and an object in motion stays in motion at constant speed unless acted upon by an external net force
Great! So for there to be no net force, in which direction must the electrostatic force be directed? Also, how big would that force be? (not looking for a number here)
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it would be acting upwards and it would be equal to that of the weight force
You’ve got it.
Final part: is the oil drop negatively or positively charged? (Remember the plate above is positively charged and the plate below the oil drop is negatively charged)
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it would be negatively charged I think?
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Yes, good job. For the electrostatic force to be upwards (opposing weight) the oil drop would have to be negatively charged so it would be repelled from the negativly charged plate below, and attracted to the positively charged plate above.
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Thank you so much for once again helping me get through tough or just confusing questions like these!! You’re a legend!
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You did all the heavy lifting, well done! I just pointed you in the right direction
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And I’m very grateful for you being able to point me in the right direction and being able to explain it in a way that I understand!
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